Method for adjusting light emitting device

ABSTRACT

Light with a short pulse width is emitted using a simple structure. A light source  101 , a differentiation circuit  102 , and a switch  103  are connected in series. When the switch  103  is switched on, inrush current flows in a capacitor  102   b  forming the differentiation circuit  102 , and accordingly the light source  101  is supplied with electric current and thereby emits light. When the capacitor  102   b  is charged, electric current flows in a resistor  102   a , and voltage drops at the resistor  102   a . Then, the voltage applied to the light source  101  is decreased, whereby the light source  101  stops emitting light. The values of the resistor  102   a  and the capacitor  102   b  are adjusted so that the above phenomenon occurs.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a method for adjusting a light emittingdevice that can perform emission of pulsed light.

2. Background Art

In technology using laser light for measuring distances, various typesof machining, etc., a short pulsed laser light with a pulse width ofseveral tens to not more than several hundreds of picoseconds isdesired. For example, in distance measurements using pulsed laser light,the precision of measuring distances can be improved by using pulsedlight with a narrow pulse width. As a technique for generating laserlight with a narrow pulse width, techniques disclosed in, for example,Japanese Examined Patent Application Publication No. 7-109911, JapaneseUnexamined Patent Application Laid-Open No. 55-107282, and JapaneseUnexamined Patent Application Laid-Open No. 2002-368329 are publiclyknown.

FIG. 1 shows a relationship between driving current and output light ofa laser diode (LD). As shown in FIG. 1, when driving current is appliedto the LD, a phenomenon may occur in which pulse-like light is emittedat first, the emitted light gradually decreases in the amplitude whileoscillating, and light with a constant intensity is finally emitted.This phenomenon can be observed in typical laser diodes. The aboveoscillation in light intensity, which occurs in the initial stage of thelight emission and gradually decreases, is called “relaxationoscillation”.

In order to perform emission of light with a short pulse width, a methodof applying driving current for a short period and performing emissionof only initial pulsed light, as shown in FIG. 2, may be used. However,it is difficult to generate a driving current having a time length onthe order of several tens to several hundreds of picoseconds using asimple circuit.

For example, a sine wave and a square wave of a frequency ofapproximately 200 MHz are easily obtained by using commercial ICs.However, for example, considering that the cycle of a high frequency of100 MHz is 10⁻⁸ seconds (10 nanoseconds=10,000 picoseconds), it is noteasy to obtain pulse-like driving current having a time length on theorder of several tens to several hundreds of picoseconds.

The pulse-like driving current having a time length on the order ofseveral tens to several hundreds of picoseconds can be obtained by usinga high frequency technique in the microwave band. For example, a signalof several tens of GHz can be obtained by an oscillator using a YIGresonator, and the pulse-like driving current having a time length onthe order of several tens to several hundreds of picoseconds can beobtained by utilizing this technique. Nevertheless, the high frequencytechnique in the microwave band may not be suitable for use for, forexample, outdoor measuring devices and the like, in view of circuitscale, production cost, need for complicated adjustment, and largeconsumption of electric power.

SUMMARY OF THE INVENTION

In view of these circumstances, an object of the present invention is toprovide a technique for performing emission of light with a short pulsewidth using a simple structure.

A first aspect of the present invention provides a method for adjustinga light emitting device including a light source, a capacitive reactancecircuit, a resistance circuit, a differentiation circuit formed of thecapacitive reactance circuit and the resistance circuit, which areconnected in parallel, and a switching element. The light sourcegenerates relaxation oscillation immediately after electric current isapplied thereto for driving light emission. The capacitive reactancecircuit exhibits low impedance immediately after the electric current isapplied and is charged by the electric charge. The resistance circuitdischarges the electrical charge charged in the capacitive reactancecircuit after a predetermined time passes after the electric current isapplied. The light source and the differentiation circuit are connectedin series. The switching element is configured to switch on or off forapplication of voltage to the light source and the differentiationcircuit. The method includes adjusting characteristics of the capacitivereactance circuit and the resistance circuit so that one of theoscillations of the relaxation oscillation is obtained.

The phenomenon in which the relaxation oscillation occurs immediatelyafter an electric current is applied for driving light emission ispublicly known as typical characteristics of lasers, and in particular,laser diodes (LDs) are mentioned as light sources in which thisphenomenon can easily occur. The capacitive reactance circuit, whichexhibits low impedance immediately after the electric current isapplied, has a function of storing electrical charge and generatesinrush current when storing the electrical charge. As the capacitivereactance circuit, a capacitor element is described. A capacitancebetween wirings or conductor patterns may also be used instead of thecapacitor element.

As the resistance circuit which discharges electrical charge charged inthe capacitive reactance circuit after a predetermined time passes afterthe electric current is applied, a variety of resistance elements, aresistor using wiring, and a resistor using a variety of conductors orsemiconductors may be described. The voltage-current characteristic ofthe resistance circuit need not necessarily be linear. For example, asthe resistance circuit, a non-linear element such as a diode, and athree-terminal element such as an FET in which bias is appropriatelyset, may also be used.

The switching element is an element by which switching on or off of acircuit, that is, a conducting condition or a non-conducting conditioncan be selected. As the switching element, a semiconductor switch suchas a bipolar transistor, an FET, or the like, may be described. Inaddition, as the switching element, an IC having a switching functionmay also be used. Alternatively, a two-terminal element, in which thecondition is changed from non-conduction to conduction when a voltage ofa threshold value or higher is applied, may also be used as theswitching element. In this case, switching on or off of the circuit iscontrolled by changing power-supply voltage. The light source and thedifferentiation circuit are serially connected in a direct manner, butthey can be serially connected via other circuits or devices.

According to a second aspect of the present invention, in the firstaspect of the present invention, at least one of capacitance value andimpedance of the capacitive reactance circuit may be adjusted so thatelectric current flows in the light source at not less than a thresholdcurrent value for oscillation immediately after the electric current issupplied to the capacitive reactance circuit and so that the electriccurrent flows in the light source at an amount at which the lightemission of the light source does not come to be an oscillationstationary state.

According to a third aspect of the present invention, in the secondaspect of the present invention, resistance value of the resistancecircuit may be adjusted to be not less than a value, at which electriccurrent approximately equal to the threshold current value foroscillation of the light source flows and at which the light source doesnot emit light, and not more than a value, at which a difference betweenvoltage dropped at the resistance circuit and a power-supply voltage isapproximately equal to a forward direction voltage of the light source.

According to a fourth aspect of the present invention, in the thirdaspect of the present invention, the capacitance of the capacitivereactance circuit may be increased whereas the resistance value of theresistance circuit may be decreased in order to increase a peak value ofthe emitted light, and the capacitance of the capacitive reactancecircuit may be decreased whereas the resistance value of the resistancecircuit may be increased in order to decrease the peak value of theemitted light.

According to a fifth aspect of the present invention, in any one of thefirst to the fourth aspects of the present invention, thecharacteristics of the capacitive reactance circuit and the resistancecircuit may be adjusted so that light of only initial oscillation of therelaxation oscillation is emitted.

According to the present invention, a technique for performing emissionof light with a short pulse width using a simple structure is obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a relationship between driving current andintensity of emitted light of a laser diode (LD).

FIG. 2 is a diagram showing a relationship between driving current andintensity of emitted light of a laser diode (LD).

FIG. 3 is a block diagram and a circuit diagram of a light emittingdevice of an embodiment.

FIG. 4 is a circuit diagram of an embodiment, and graphs (a) to (c)showing changes in voltage at each part.

FIG. 5 is a conceptual diagram for explaining operation of adifferentiation circuit.

FIGS. 6A and 6B are views showing waveforms of emitted light.

FIG. 7 is a view showing a waveform of emitted light.

FIG. 8 is a view showing a waveform of emitted light.

FIGS. 9A and 9B are circuit diagrams of embodiments.

FIGS. 10A to 10C are circuit diagrams of embodiments.

FIG. 11 is a block diagram of an embodiment.

FIG. 12 is a view showing waveforms of emitted light.

FIG. 13 is a view showing a waveform of emitted light.

FIG. 14 is a view showing waveforms of emitted light.

FIGS. 15A and 15B are block diagrams of light emitting devices ofembodiments.

FIG. 16 is a block diagram of a light emitting device of an embodiment.

PREFERRED EMBODIMENTS OF THE INVENTION 1. First Embodiment Structure

FIG. 3 shows a block diagram and a circuit diagram of a light emittingdevice 100 of an embodiment. The light emitting device 100 includes alight source 101, a differentiation circuit 102, and a switch 103, whichare connected in series, and power-supply voltage V is applied betweenboth ends of this serial connection. In this example, the light source101 is a laser diode (LD), the differentiation circuit 102 is a circuitformed by connecting a resistor R (102 a) and a capacitor C (102 b) inparallel, and the switch 103 is an FET.

Regarding circuit constants, for example, the power-supply voltage is5V, the resistance value of the resistor 102 a is 200 ohm, thecapacitance of the capacitor 102 b is 20 pF, and a square wave with afrequency of approximately several tens of MHz at most is used as acontrol signal.

Operation

When the FET is switched off (there is no electric conduction betweensource and drain), the voltage V is not applied to the light source 101and to the differentiation circuit 102, and the light source 101 doesnot emit light. When a control signal is applied to a gate electrode ofthe FET, and the switch 103 is switched on, electrical charge flows inthe capacitor 102 b, and inrush current is generated, whereby drivingcurrent flows in the light source 101. The driving current makes thelight source 101 emit light.

As the electrical charge accumulates in the capacitor 102 b, the inrushcurrent is suddenly decreased. According to the decrease of the inrushcurrent at the capacitor 102 b, electric current flowing into theresistor 102 b is increased. When the capacitor 102 b is completelycharged with the electrical charge, the electric current flowing intothe capacitor 102 b stops, and the value of the electric current flowingin the resistor 102 a and the value of the electric current flowing inthe light source 101 become the same.

Here, the value of the power-supply voltage (+V) and the values of thecapacitor 102 b and the resistor 102 a are set so that the chargingperiod (inrush current flowing period) of the capacitor 102 b isapproximately the same as the length of Δt in FIG. 2. When the inrushcurrent stops flowing into the capacitor 102 b, the voltage drops at theresistor 102 a, thereby decreasing the voltage applied to the lightsource 101. The value of the resistor 102 a and the other parameters areselected so that the light source 101 stops emitting light by thisdecrease in the voltage. In other words, when the inrush current stopsflowing into the capacitor 102 b, electric current flows in the resistor102 a, whereby the resistor 102 a controls (limits) electric currentflowing into the light source 101. Then, it can also be understood thatthe value of the resistor 102 a and the other parameters are set so thatthe value of LD current flowing in the light source 101 is made lessthan a threshold value for light emission by this electric currentcontrol.

According to these structures and the setting of the parameters, thelight source 101 emits light while the capacitor 102 b is charged, andimmediately after that, voltage drops (electric current is limited) atthe resistor 102 a. Then, the voltage applied to the light source 101(the value of electric current flowing in the light source 101) isdecreased, whereby the light source 101 stops emitting light. As aresult, the generation of subsequent relaxation oscillation, as shown inFIGS. 1 and 2, is suppressed, and only the initial pulse of light isemitted.

The above operation will be described in more detail with reference tothe figures hereinafter. FIG. 4 shows graphs (a) to (c) showing changesin voltage applied at each part. The graph (a) shows a voltage Vcs inthe light source 101 side of the differentiation circuit 102. Thevoltage of V-Vcs is the voltage to be applied to the light source 101.The graph (b) shows a voltage to be applied to the differentiationcircuit 102, that is, a voltage Vdif to be applied to the resistor 102 aand the capacitor 102 b. The graph (c) shows a voltage Vd to be appliedbetween the source and the drain of the switch 103. FIG. 5 is aconceptual diagram showing operating conditions of the differentiationcircuit 102 as time passes from (a) to (d).

When the time is (a) in FIG. 5, the switch 103 is in a condition beforebeing switched on (that is, it is in a condition of being switched off).In this stage, the power-supply voltage V is not applied to the lightsource 101 and the differentiation circuit 102. After the switch 103 isswitched on, the capacitor 102 b starts to charge, and inrush currentflows in the capacitor 102 b. In the first stage, the capacitor 102 b iselectrically shorted, and flowing electric current primarily includesthe inrush current into the capacitor 102 b (refer to the view at time(b) in FIG. 5). By making the inrush current flow into the capacitor 102b, driving current flows in the light source 101, and the light source101 emits light.

Then, as the capacitor 102 b is gradually charged, the resistance valueof the capacitor 102 b is gradually increased, and electric currentflowing in the resistor 102 a is increased. This condition is shown inthe views at times (b) and (c) in FIG. 5. In the stage at the time (b)in FIG. 5, the voltage Vdif applied to the differentiation circuit 102is gradually increased. This is because the voltage drops at theresistor 102 b.

While the inrush current at the capacitor 102 b is generated (the periodindicated by the reference numeral 104 in the graph (a) in FIG. 4), thepotential of the electrode in the switch 103 side of the capacitor 102 bis gradually decreased (gradually decreased to ground potential),whereby the value of Vd fluctuates as shown in the graph (c) in FIG. 4.

According to decrease in the inrush current at the capacitor 102 b, theelectric current flowing into the resistor 102 a is increased, and thevoltage drops at the resistor 102 a, whereby the voltage applied to thelight source 101 is decreased. In other words, the effect of the voltagedrop generated at the resistor 102 a is increased with respect to thepower-supply voltage V, and the voltage applied to the light source 101is decreased accordingly. The resistance value of the resistor 102 a andthe other parameters are set so that the voltage applied to the lightsource 101 at that time is less than the threshold value for the lightemission. That is, the resistance value of the resistor 102 a and theother parameters are set so that the voltage applied to the light source101 becomes less than the threshold value for the light emission by thevoltage drop generated at the resistor 102 a in the period 105 shown ingraph (a) in FIG. 4.

Thus, the light source 101 emits light during the period 104 and stopsemitting light during the period 105. Here, the values of CR of thedifferentiation circuit 102 are adjusted so that the length of theperiod 104 approximately corresponds to the length of the period Δtshown in FIG. 2, whereby only one pulse of light at the initial stage isemitted.

In this condition (period 105), when the switch 103 is switched off, theelectric current stops flowing into the light source 101, and theelectrical charge charged in the capacitor 102 b flow in the resistor102 a and are consumed. Thus, the condition returns to the initialcondition in which the switch 103 is switched off.

Then, by switching on the switch 103 again, the operation in the samemanner as described above is repeated, and a second pulse of light isemitted by the light source 101. Accordingly, by repeating switching onand off of the switch 103, the light source 101 is made to emit a pulseof light repeatedly. Setting of CR of Differentiation Circuit

In order to apply voltage at or above a threshold value to the lightsource 101 during only the period Δt in FIG. 2 as described above,selecting appropriate values of CR of differentiation circuit 102 isimportant. The method of selecting the values of the CR of thedifferentiation circuit 102 will be described hereinafter.

In order to generate pulsed light, it is necessary to make electriccurrent flow in the light source (laser diode, LD) 101 at an amount inthe range for generating the phenomenon as follows. The lower limit ofthe electric current is the current value at which the light source 101starts to emit light (a threshold current value for oscillation), andthe upper limit is the current value at which only the initial pulse ofthe relaxation oscillation is generated. In order to generate the LDcurrent within this range, the constants of the C and the R forming thedifferentiation circuit must be adjusted.

First, as an initial condition of the circuit, a condition is assumed inwhich there is no electrical charge in the capacitor 102 b and theswitch 103 is switched off. In this condition, since electric currentdoes not flow in the resistor 102 a and in the capacitor 102 b, electriccurrent also does not flow in the light source (LD) 101. Naturally, thelight source 101 does not emit light.

When the switch 103 is switched on, voltage is suddenly applied to theresistor 102 a and the capacitor 102 b. Since this sudden change in thevoltage has a high frequency component, electric current flows into thecapacitor 102 b having lower alternating current impedance than theresistor 102 a. This phenomenon can also be understood such that inrushcurrent flows into the capacitor 102 b, to which application of the DCvoltage is started, according to the accumulation of electrical chargetherein. Since the capacitor 102 b and the resistor 102 a are connectedin parallel, immediately after the switch 103 is switched on, theelectric current flowing into the capacitor 102 b is the same as theelectric current flowing into the light source 101 and makes the lightsource 101 emit light. The capacitor 102 b is gradually charged withelectrical charge as time passes, and the electric current stops flowinginto the capacitor 102 b when the amount of the accumulated electricalcharge reaches the capacitance of the capacitor 102 b. The voltage drops(electric current is limited) at the resistor 102 b at this time,whereby the light source 101 is made to stop light emission even thoughthe switch 103 is switched on.

It can be understood from the above description that electric currentflows in the light source 101 due to the inrush current at the capacitor102 b, which is generated while the electrical charge accumulates in thecapacitor 102 b. Therefore, by appropriately setting the value of thecapacitance of the capacitor 102 b, the amount of the electric currentflowing in the light source 101 is determined, whereby electric currentin the range necessary for emitting a pulse of light is obtained.

While the above phenomenon occurs, as the electrical charge accumulatesin the capacitor 102 b, the voltage applied to the capacitor 102 b isincreased. Since the capacitor 102 b and the resistor 102 a areconnected in parallel, the voltages at the capacitor 102 b and at theresistor 102 a are equal in value. Therefore, when the voltage appliedto the capacitor 102 b is increased, the electric current flowing in theresistor 102 a is also increased. This electric current flowing in theresistor 102 a makes the voltage drop at the resistor 102 a, whereby thevoltage applied to the light source 101 is decreased.

Since the LD current does not flow when the voltage is lower than aforward direction voltage of the LD, the resistor 102 a functions tolimit the LD current. Therefore, by appropriately selecting the value ofthe resistor 102 a, the following condition is obtained. That is, the LDcurrent flows and makes the light source 101 emit light when the valueof the electric current flowing in the resistor 102 a is small, and theLD current stops flowing and the light source 101 stops light emissionwhen the value of the electric current flowing in the resistor 102 a islarge.

In cases of performing continuous emission of pulses of light, theelectrical charge charged in the capacitor 102 b must be discharged soas to make the LD current flow in the light source 101 again. Thisdischarge is performed by the resistor 102 b that is connected with thecapacitor 102 b in parallel. Specifically, when the switch 103 isswitched off, the resistor 102 a connected with the capacitor 102 b inparallel discharges the electrical charge charged in the capacitor 102b, and the circuit shown in FIG. 3 returns to the initial condition.Accordingly, the resistor 102 a has a first function of limiting the LDcurrent and suppressing subsequent relaxation oscillation so that singlepulsed light is emitted and has a second function of discharging theelectrical charge charged in the capacitor 102 b and making the circuitreturn to the initial condition for the next pulsed light emission.

The indication for setting the values of the capacitor C (102 b) and theresistor R (102 a) is described as follows.

(1) The minimum value of C: Capacitance value so that electric currentflows in the LD at not less than the threshold current value foroscillation.

-   -   (If it is too small, the LD does not oscillate.)        (2) The maximum value of C: Capacitance value so that electric        current flows for generating only an initial pulse in the        relaxation oscillation and suppressing subsequent pulses        (depending on the value of R).    -   (If it is too large, continuous relaxation oscillation occurs.)        (3) The minimum value of R: Resistance value so that the value        of the LD current is the same as the threshold current value for        oscillation.    -   (If it is too small, continuous relaxation oscillation occurs.)        (4) The maximum value of R: Resistance value so that a        difference between the voltage dropped at the R and the        power-supply voltage is the same as the value of the forward        direction voltage of the LD (depending on the value of C).    -   (If it is too large, the discharge is not completed before the        next pulse generation pulse.)

In addition, the peak value of the pulsed light and the values of C andR have approximately the following relationships.

-   -   In order to increase the intensity of the light pulse, the value        of C is increased, while the value of R is decreased.    -   In order to decrease the intensity of the light pulse, the value        of C is decreased, while the value of R is increased.

It should be noted that the values of C and R may not be uniquelydetermined because there is a correlation between C and R. In addition,the values of C and R must be selected in consideration of the effect ofsuppressing the subsequent relaxation oscillation.

Since the peak value of the pulsed light is far more affected by thevalue of C than by the value of R, the value of the C should beapproximately adjusted, whereas the value of R should be finelyadjusted.

Specific examples will be described hereinafter. FIG. 6A shows a case inwhich plural pulses are generated instead of a single pulse, that is, acase in which continuous relaxation oscillation occurs. FIG. 6B shows acase in which a single pulse is generated, but there can be seen a smalleffect of the subsequent relaxation oscillation. The phenomena as shownin FIGS. 6A and 6B occur when the value of the LD current is too large.Therefore, in order to reduce the value of the LD current, the value ofC is decreased, and then the value of R is finely adjusted. It should benoted that there may be cases in which the peak of the emitted light issmall, and the light may not even be emitted, if the value of C is madetoo small.

FIG. 7 shows a case, in which the emitted light has a small peak and awide pulse width. In such a case, the value of the LD current is toosmall, and therefore, the value of C is increased, and then the value ofR is finely adjusted. For reference, an example of a waveform of apulsed light when the values are adjusted appropriately is shown in FIG.8.

Advantages

According to the above structure, the period of emitting light by thelight source 101 (width of the emitted pulsed light) can be shorter thanthe period during which the switch 103 is switched on. Specifically,even when the control signal for driving the switch 103 has a wide pulsewidth, and the switch 103 is thereby switched on for a long period, thelight source 101 is made to stop emitting light when the capacitor 102 bis charged completely. This is because the voltage applied to the lightsource 101 is decreased compared to the initial level and becomes lowerthan the threshold voltage due to the voltage drop occurring at theresistor 102 a at that timing. That is, the light source 101 is made toemit light by using a phenomenon in which the inrush current at thecapacitor 102 b occurs, whereby the light source 101 can be made to emitlight for an extra short period even when the switch 103 is switched onfor a long period.

In other words, by using the inrush current occurring while thecapacitor 102 b is charged and by using the voltage drop occurring atthe resistor 102 a according to the decrease in the inrush current, thelight source 101 can be made to emit a pulse of light with a pulse widthshorter than that of the control signal for controlling the switch 103even when the control signal has a low frequency. Therefore, forexample, even in cases of generating a pulse of light with a pulse widthon the order of several tens to several hundreds of picoseconds, thecontrol signal for controlling the switch 103 need not have a pulsewidth on the order of several tens to several hundreds of picoseconds.

The above operation can be understood as a phenomenon in which theelectric current is increased at the resistor 102 a as the charge in thecapacitor 102 b progresses and is thereby limited by the resistor 102 a,whereby the electric current flowing into the light source (LD) 101 isdecreased, and the subsequent relaxation oscillation is suppressed.According to the experiments and computer simulation conducted by theinventors of the present invention, it was confirmed that the pulsewidth of the light emitted by the light source 101 can be approximatelyseveral tens to several hundreds of picoseconds. It should be noted thatthe pulse width of the emitted pulsed light is not limited to the orderof several tens to several hundreds of picoseconds.

As shown in FIG. 3, the circuit for generating pulsed light using thepresent invention is very simple and can be made so as to be small andlight in weight and to consume low levels of electrical power, and at alow production cost. The control signal for controlling the function ofthe switch 103 can be obtained by using a commercial IC (such as a FPGAin which a signal generating circuit is formed), and no expensive,complicated oscillating circuit is necessary.

In addition, according to the structure shown in FIG. 3, the peak valueof the generated pulsed light is not susceptible to variation in thepower-supply voltage. In general, when the power-supply voltage isdirectly applied to the LD, the intensity of emitted light is greatlyaffected by the variation and the deviation of the power-supply voltage.Therefore, stabilization of the power supply must be taken intoconsideration. However, compared with cases of directly connecting thepower supply, the effect of the power-supply voltage on the intensity ofemitted light is small in cases of using the inrush current at thecapacitor as the LD current. The detailed reasons for this phenomenonhave not been clear, but it can be thought that, by using the inrushcurrent at the capacitor as the LD current, the electric current flow tothe LD is affected primarily by the capacitor, and the effect of thepower supply is relatively decreased.

Other Matters

Although an example of using a positive power supply is shown in FIG. 3,an embodiment operated by a negative power supply can also be used. Whena negative power supply is used, the light source 101 side is grounded,and the switch 103 side is connected with the negative power supply(−V). In addition, although an FET is used as the switch 103 in FIG. 3,a bipolar transistor or another switching element may be used. Moreover,an element functioning as a capacitor may be used as C instead of anormal capacitor element. Furthermore, an element functioning as aresistance (for example, an FET to which a bias is applied, or the like)may be used as R instead of a normal resistance element.

FIGS. 9A and 9B and FIGS. 10A to 10C show modified examples of thecircuit structure shown in FIG. 3. The circuits shown in FIGS. 9A and 9Band FIGS. 10A to 10C will be describe hereinafter. FIGS. 9A and 9B andFIGS. 10A to 10C show examples of cases using negative power supplies.

FIG. 9A shows an example of a case using a variable capacitor as thecapacitor of the differentiation circuit. In the example shown in FIG.9A, a variable capacitor is used as the capacitor of the differentiationcircuit, whereby the capacitance of the differentiation circuit iseasily adjusted. FIG. 9B shows a structure in which multiple capacitorsare connected in parallel via switches, and the capacitance is adjustedby switching the switches on and off. As the plural capacitors,capacitors having different capacitance from each other may be used, butcapacitors having the same capacitance can also be used. In the exampleshown in FIG. 9B, by changing the combination of the capacitors, thecapacitance of the differentiation circuit is easily adjusted.

FIG. 10A shows an example of using a variable resistor as the resistorof the differentiation circuit. FIG. 10B shows an example of using avariable resistor, which can be varied by a control signal, as thevariable resistor shown in FIG. 10A. By forming the circuit structure asshown in FIG. 10A or 10B, the resistance value of the resistor formingthe differentiation circuit is easily adjusted. FIG. 10C shows anexample of connecting a thermistor with the resistor forming thedifferentiation circuit in parallel. By using the thermistor,temperature characteristics of the light source can be compensated for.

As an example of a structure in which the resistance value isadjustable, a structure, in which plural resistors are connected inparallel via switches, as shown in FIG. 9B, and the resistance value isset by selecting the switches to be switched on, may be described. Inaddition, by combining this structure with the structure shown in FIG.9B and by selecting the switches to be switched on, both the resistancevalue of the resistor and the capacitance of the capacitor of thedifferentiation circuit can be adjustable. As the plural resistors,resistors having different resistance from each other may be used, butresistors having the same resistance can also be used.

2. Second Embodiment

The intensity of light emitted by the laser diode (LD) istemperature-dependent. Here, an example of a structure for decreasingvariation of the intensity of emitted light of the light source due totemperature change will be described. FIG. 11 shows a block diagram ofthis embodiment. FIG. 11 shows a light emitting device 200 including avariable voltage power supply 105, a temperature sensor 106, a lightsource 101, a differentiation circuit 102, and a switch 103. Here, thelight source 101, the differentiation circuit 102, and the switch 103are the same as in the First Embodiment shown in FIG. 3.

The variable voltage power supply 105 is a negative power supply andvaries the value of output voltage according to the temperature detectedby the temperature sensor 106. The negative voltage set and generated bythe variable voltage power supply 105 is applied to the circuit in whichthe light source 101, the differentiation circuit 102, and the switch103 are connected in series.

FIGS. 12 to 14 show relationships between environmental temperature,power-supply voltage, and waveform and peak value of pulsed light in acase of using the circuit structure shown in FIG. 11. As shown in FIGS.12 to 14, when the power-supply voltage is constant, the peak value ofthe pulsed light emitted by the light source 101 constructed of thelaser diode greatly varies depending on the environmental temperature.However, as shown in FIGS. 12 and 14, by changing the power-supplyvoltage according to the environmental temperature, pulses of light withapproximately the same waveform and approximately the same peak valuecan be obtained.

The variable voltage power supply 105 houses a controller using amicrocomputer. The microcomputer has a memory part that stores datatables of results of examining the relationship between theenvironmental temperature and the power-supply voltage, which arenecessary for obtaining particular peak values. Regarding the operationof the variable voltage power supply 105, the variable voltage powersupply 105 is controlled so as to output corresponding power supplyvoltage by applying the environmental temperature detected by thetemperature sensor 106 to the data tables.

Since laser diodes (LDs) are generally greatly affected by power-supplyvoltages, when the power-supply voltage is directly applied to the LD,the change in the intensity of emitted light by changing thepower-supply voltage would be difficult in view of reproducibility.However, in the case of using the inrush current at the capacitor formaking the LD emit light, the effect of the power supply is reduced,whereby adjustment of the intensity of emitted light by changing thepower-supply voltage can be performed at a high level ofreproducibility.

3. Third Embodiment

FIG. 15A shows a light emitting device 300 having a switch 103 at aposition different from the case shown in FIG. 3. In the light emittingdevice 300, the switch 103 is arranged between a light source 101 and adifferentiation circuit 102. The light emitting device 300 is an examplein which the light source and the differentiation circuit are connectedin series. The light source 101, the differentiation circuit 102, andthe switch 103 of the light emitting device 300 are the same as thosedescribed relating to the structure shown in FIG. 3.

The operation of the light emitting device 300 is the same as that ofthe light emitting device 100 shown in FIG. 3. The operation of thelight emitting device 300 will be simply described hereinafter. When theswitch 103 is switched off, no voltage is applied to the capacitor (thereference numeral 102 b in FIG. 3) of the differentiation circuit andalso, no voltage is applied to the light source 101, whereby the lightsource 101 does not emit light. When the switch 103 is switched on,voltage is applied to the capacitor (the reference numeral 102 b in FIG.3) of the differentiation circuit 102, and inrush current flows. Bymaking the inrush current flow, electric current flows in the lightsource 101, and the light source 101 emits light.

Then, when the capacitor (the reference numeral 102 b in FIG. 3) of thedifferentiation circuit 102 is charged, electric current flows in theresistor (reference numeral 102 a in FIG. 3) that is connected with thecapacitor in parallel, and the voltage drops (electric current islimited) at the resistor, whereby the light source 101 stops emittinglight. Thus, light of only the initial pulse in the relaxationoscillation is emitted.

FIG. 15B shows a light emitting device 400 having a switch 103 at aposition different from the cases shown in FIGS. 3 and 15A. In the lightemitting device 400, the switch 103 is arranged between a light source101 and a positive power supply, and the light source 101 is seriallyconnected with a differentiation circuit 102 arranged at the groundside. The light source 101, the differentiation circuit 102, and theswitch 103 of the light emitting device 400 are the same as thosedescribed relating to the structure shown in FIG. 3. The operation ofthe light emitting device 400 is the same as those of the light emittingdevice 100 shown in FIG. 3 and the light emitting device 300 shown inFIG. 15A.

4. Fourth Embodiment

FIG. 16 shows a distance measuring device 500. The distance measuringdevice 500 is a device for measuring a distance to an object to bemeasured using laser light and includes a light emitting device 100, anilluminating part 501, a light receiving part 502, a signal processor503, and a display part 504.

The light emitting device 100 has the structure shown in FIG. 3.Naturally, another light emitting device exemplified in the presentspecification can also be used. The illuminating part 501 includes anoptical system for illuminating the object with laser light output fromthe light emitting device 100. The light receiving part 502 includes anoptical system and a light receiving element (such as a photo diode orthe like) and receives light, which is emitted from the illuminatingpart 501 and is reflected by the object. The signal processor 503calculates a distance to the object based on the detected light receivedby the light receiving part 502. The calculation performed in the signalprocessor 503 is the same as that in an ordinary laser distancemeasuring device. The display part 504 is a displaying device such as aliquid crystal display or the like and displays the distance to theobject, which is calculated by the signal processor 503.

The distance measuring device 500 uses distance measuring light with ashort pulse width generated by the light emitting device 100, wherebythe distance is measured with high precision. In addition, the lightemitting device 100 has a simple structure and consumes low levels ofelectric power, and it can be obtained at low production cost, wherebythe distance measuring device 500 can be made so as to be small and toconsume low levels of electric power, at a low production cost.

Although the laser distance measuring device is exemplified as anexample of utilizing the light source of the present invention in thisembodiment, the light source of the present invention, which emitspulsed light using the differentiation circuit, can be applied invarious types of devices using pulsed light (for example, a lasermachining device or the like).

What is claimed is:
 1. A method for adjusting a light emitting devicecomprising: a light source that generates a relaxation oscillationimmediately after an electric current is applied thereto for drivinglight emission; a capacitive reactance circuit that exhibits lowimpedance immediately after the electric current is applied thereto andwhich is chargeable with electrical charge; a resistance circuit thatdischarges electrical charge charged in the capacitive reactance circuitafter a predetermined time passes after the electric current is applied;a differentiation circuit formed of the capacitive reactance circuit andthe resistance circuit, which are connected in parallel; and a switchingelement, wherein the light source and the differentiation circuit areconnected in series, and the switching element is configured to switchon or off for application of voltage to the light source and thedifferentiation circuit, the method comprising adjusting characteristicsof the capacitive reactance circuit and the resistance circuit so thatone of the oscillations of the relaxation oscillation is obtained. 2.The method for adjusting the light emitting device according to claim 1,wherein at least one of capacitance value and impedance of thecapacitive reactance circuit is adjusted so that electric current flowsin the light source at not less than a threshold current value foroscillation immediately after the electric current is supplied to thecapacitive reactance circuit and so that the electric current flows inthe light source at an amount at which the light emission of the lightsource does not come to be an oscillation stationary state.
 3. Themethod for adjusting the light emitting device according to claim 2,wherein resistance value of the resistance circuit is adjusted to be notless than a value, at which electric current approximately equal to thethreshold current value for oscillation of the light source flows and atwhich the light source does not emit light, and not more than a value,at which a difference between voltage dropped at the resistance circuitand a power-supply voltage is approximately equal to a forward directionvoltage of the light source.
 4. The method for adjusting the lightemitting device according to claim 3, wherein the capacitance of thecapacitive reactance circuit is increased whereas the resistance valueof the resistance circuit is decreased in order to increase a peak valueof the emitted light, and the capacitance of the capacitive reactancecircuit is decreased whereas the resistance value of the resistancecircuit is increased in order to decrease the peak value of the emittedlight.
 5. The method for adjusting the light emitting device accordingto claim 1, wherein the characteristics of the capacitive reactancecircuit and the resistance circuit are adjusted so that light of onlyinitial oscillation of the relaxation oscillation is emitted.